Airtime complaint

I had a bad experience , I bought airtime last Sunday to my surprise the airtime was loaded already the previous day..... because when I was trying to loading it kept on saying invalid pin for about 3 times up until I got barred. I went back to the shop....the guy that served me told me there's nothing he can do I must call the service provider which is MTN I walked out of the shop and I was sooooo upset....to cut the story short I phoned MTN only to find out the airtime was loaded already on Saturday so they sold me used airtime. I was advised to go back to the store i couldn't and I don't want the refund but jus to give you guys a lil advice.....such petty issues need to be taken care of with a professional attitude. Thank you.